Molecular drag pumping mechanisms operate on the general principle that, at low pressures, gas molecules striking a fast moving surface can be given a velocity component from the moving surface. As a result, the molecules tend to take up the same direction of motion as the surface against which they strike, which urges the molecules through the pump and produces a relatively higher pressure in the vicinity of the pump exhaust.
These pumping mechanisms generally comprise a rotor and a stator provided with one or more helical or spiral channels opposing the rotor. Types of molecular drag pumping mechanisms include a Holweck pumping mechanism comprising two co-axial cylinders of different diameters defining a helical gas path therebetween by means of a helical thread located on either the inner surface of the outer cylinder or on the outer surface of the inner cylinder, and a Siegbahn pumping mechanism comprising a rotating disk opposing a disk-like stator defining spiral channels that extend from the outer periphery of the stator towards the centre of the stator. Another example of a molecular drag pumping mechanism is a Gaede mechanism, whereby gas is pumped around concentric channels arranged in either a radial or axial plane. In this case, gas is transferred from stage to stage by means of crossing points between the channels and tight clearance ‘stripper’ segments between the adjacent inlet and outlet of each stage. Siegbahn and Holweck pumping mechanisms do not require crossing points or tight clearance ‘stripper’ segments because their inlets and outlets are disposed along the channel length.
For manufacturing purposes a Siegbahn pumping mechanism may be preferred to the Holweck and Gaede pumping mechanisms. However, for a given rotor-to-stator clearance, a Siegbahn pumping mechanism typically requires more pumping stages to achieve the same levels of compression and pumping speed as a Holweck pumping mechanism. Furthermore, a Siegbahn pumping mechanism requires tight clearances to be achieved in an axial direction, otherwise more pumping stages—and thus greater power consumption—will be required to achieve the required level of pumping performance. Achieving tight axial clearances between the rotor and stator components of a Siegbahn pumping mechanism can be relatively difficult and/or costly. For example, U.S. Pat. No. 6,585,480 describes a vacuum pump comprising a drive shaft having a plurality of rotor disks of a Siegbahn pumping mechanism mounted along the length of the shaft. Stator disks extend radially inwardly from the stator of the vacuum pump and are located between the rotor disks. A relatively complex and expensive magnetic bearing arrangement comprising upper and lower radial magnetic bearings, and an axial magnetic bearing, is provided for supporting the drive shaft out of contact with the stator, and for maintaining the required axial clearances between the rotor and stator disks.